1. Field of the Invention
The present invention relates to an electric power converting device, and more particularly, to an electric power converting device and power converting method for controlling doubly-fed induction generators, which provides a synchronous generator for generating auxiliary electric power independently of a doubly-fed induction generator so as to generate electricity even in a system power-free environment, a grid-side converter is composed of a three-phase four-wire converter so as to generate a balanced voltage even in an unbalanced load condition and automatically synchronize a stator voltage of a doubly-fed induction generator and a system voltage with each other.
2. Background of the Related Art
In general, a power converter refers to a device in which refines a low-quality primary energy having variable voltage and variable frequency characteristics caused by a variable wind speed or tidal current speed into a high-quality secondary energy having constant voltage and constant frequency characteristics so as to be linked to a power system in a new & renewable energy power generation system using wind power, tidal current power, etc.
Particularly, a generator most widely applied in a new & renewable energy power generation system market is a doubly-fed induction generator (DFIG) which features that a rotor winding is additionally provided to a cage-type generator so as to control slip power using the power converter.
FIG. 1 is a block diagram illustrating the construction of a power converter for a doubly-fed induction generator including an AC-to-DC converter and a DC-to-AC converter according to the prior art, FIG. 2 is a circuit diagram illustrating the construction of a power converter manufactured by Vestas Wind System according to the prior art, and FIG. 3 is a circuit diagram illustrating the construction of a power converter for a doubly-fed induction generator including two three-phase three-wire converters according to the prior art.
Referring to FIG. 1, it can be seen that the structure of the power converter used in the doubly-fed induction generator has a AC-DC-AC conversion function in which AC power is converted into DC power which in turn is converted into AC power.
Referring to FIG. 2, a grid-side converter for controlling the doubly-fed induction generator of the Vestas Wind System proposed in U.S. Pat. No. 6,856,040 B2 is configured of a three-phase bridge diode to have a unidirectional property.
However, such a configuration entails a problem in that it can recover energy from the system but cannot recover (generate) energy from the system using the grid-side converter.
In addition, this configuration encounters a structural problem in that an active power and a power factor of a stator side of the doubly-fed induction generator cannot be controlled directly. Specifically, the power factor is controlled by a passive capacitor, and energy transferred to the system employs a heat consumption method using a damping resistance, which makes it difficult to properly control the active power and the power factor of the stator side. Also, 20% or so of the amount of electricity generation is recovered from the rotor winding and this recovered energy is consumed as heat, which leading to a degradation of efficiency.
Referring to FIG. 3, to address and solve the above problems, U.S. Pat. No. 5,798,631A has proposed a 3-leg IGBT or 3-leg IGCT structure which includes a three-phase three-wire converter to allow the grid-side converter to recover energy bi-directionally.
Such a structure controls an active power and a power factor recovered from a stator side of the doubly-fed induction generator irrespective of the speed and load condition, and controls the power factor to be maintained at 1 so that 20% of energy recovered from a rotor side of the doubly-fed induction generator is recovered from the system and simultaneously the recovered energy is maximized.
Further, this structure controls a voltage applied across a capacitor connected between a generator-side converter and a grid-side converter to be always maintained at a constant magnitude so as to allow energy generated through the rotor winding to be transferred to the system.
However, the above structure also has a problem in that in case where the system is unstable and instantaneous power interruption occurs except an ordinary state, the generated voltage is not in a three-phase balanced state. That is, when the capacity of the system is very large and the capacity of the distributed generation is small, no problem occurs. But, there is caused a problem in that since the above-mentioned control properties cannot be maintained in an unbalanced condition (load, power supply, etc.), the new & renewable energy power generation system (wind power/tidal current/tidal current power/wave power generation system) is stopped.
Furthermore, the above structure entails a problem in that it can be used only in an environment where the system voltage exists since the power system voltage is directly applied to the stator winding to provide a magnetic flux component and hence the above structure is usable in a system-connected condition. Moreover, the synchronization characteristic in which a stator voltage of the doubly-fed induction generator and a system voltage are synchronized with each other is sensitively affected by the installation position of an encoder.